1. Field of the Invention
Continuous monitoring of pressures from a human or animal body or body cavity requires some kind of processing of the pressure measurements. Pressure measurements within a human being or animal are created from pressure waves created by the cardiac beats, though this fact is not necessarily taken into account when measuring human or animal pressures. Pressure measurements may be derived from inside or outside a human or animal body or body cavity. It may be preferable to place a pressure sensor outside a body or body cavity, but the problem is to obtain reliable pressure measurements from such sensor locations. Depending on the type of such pressure measurements, signals related to pressure measurements may be garbled with noise, and the pressure difference from inside to outside the body cavity may be unknown. Pressure monitoring may have a more widespread role than reflected by the current and existing use. For example, various types of fluid flow valves are used to drain excess fluid from a body cavity such as a human brain or spinal fluid body cavity. Related to the function of such valve devices, pressure monitoring has no or a minimal place.
2. Related Art
Continuous pressure monitoring has a widespread use concerning arterial blood pressure monitoring, ocular bulb pressure monitoring, intracranial pressure monitoring, lumbar cerebrospinal fluid pressure monitoring, urinary tract pressure monitoring, and gastrointestinal tract pressure monitoring. Depending on how pressure measurements are performed, continuous pressure signals may be obtained. Most current and existing technologies solely use analogue signals, though modern data technology allows such analogue signals to be converted into digital data signals. Some kind of signal processing may be applied to analogue as well as digital pressure signals.
Though monitoring of pressures within a human body or body cavity has been used for many decades, it is still unclear how pressure measurements should be processed to give best possible information from said measurements. It is well known that pressures have a static component (mean pressure) and a dynamic component (pulse pressure), related to the fact that pressure waves within a human or animal body or body cavity are created by cardiac beat-induced pressure waves. During pressure monitoring usually the static component is assessed whereas the role of the dynamic component is unclear. Technologies (e.g. by using fast Fourier transformation of pressure signals) that measure the dynamic component of pressure measurements give no or minimal control as to whether the dynamic pressure changes are related to cardiac beat-induced pressure waves or not.
Continuous pressure signals are processed by computation of mean pressure, usually computed as the sum of pressure levels divided by the numbers of samples. It is not possible to evaluate whether said mean pressure is related to cardiac beat-induced pressure waves or not. According to current and prior art technology, evaluation whether pressure measurements are associated with cardiac beat-induced pressure waves is based on visual inspection of the pressure wave, or by inspection whether diastolic and systolic pressure values are different. Such evaluation may be very user-dependent and misleading. For practical purposes, continuous inspection of a pressure waveform during pressure monitoring is impossible. Furthermore, waveform analysis according to prior art technology (e.g. fast Fourier analysis or modifications thereof), does not allow assessment whether pressure waves are related to cardiac beats or not. Given bad signal quality such types of analyses can be very misleading. The inventor previously has described a method for processing continuous pressure signals in the following patent application: U.S. Ser. Nos. 10/283,245; 10/613,112; PCT/NO02/00164; and PCT/NO03/0029.
During pressure measurements it is preferable to use non-invasive sensors. The term non-invasive refers to the fact that the skin does not need to be penetrated to measure pressure within a body or body cavity. There are numerous examples of non-invasive pressure monitoring. Placing a device on the skin, thereby sensing the arterial blood pressure within the body tissue, may monitor arterial blood pressure. Transcranial Doppler may provide signals that are transformable into pressure-related signals indicative of intracranial pressure. Pressure-related signals indicative of intracranial pressure may as well be measured by means of a sensor device measuring air pressure within the outer ear channel after air-tight sealing of the outer ear by some kind of closing material to exclude interference from the atmospheric air pressure. The problem with so-called non-invasive pressure monitoring is that absolute pressure within the body or body cavity is unknown. The absolute pressure usually refers to the pressure difference between the pressures within the body or body cavity and the atmospheric pressure. Another major problem is that it can be impossible to know whether the pressure measurements are good or bad, i.e. whether the quality of the measurements or pressure signals are good or bad. This problem is at least partly related to the lack of a standard of what might be considered a good (or bad) pressure measurement.
Since the 1950's fluid flow valves have been used to drain excess fluid from a human brain or spinal fluid body cavity. Such fluid flow valves may be controllable, i.e. the degree of fluid drainage is adjustable. Such devices include some kind of mechanically adjustable valves. Pressure monitoring has received no or minimal role as related to said drainage of fluid from a brain or spinal fluid body cavity. A major cause is that pressure monitoring according to prior art technology measures absolute pressure, i.e. relative to atmospheric pressure. Changes in atmospheric pressure would change the zero pressure level and thus the measured pressure values. Sensor-related drift of zero pressure level also heavily affects the pressure measurements that are relative to atmospheric pressure.